The invention concerns a scanning head for a measuring system and a method for manufacturing and using the scanning head.
British Patent 1,504,691 and a corresponding German Patent Application 25 11 350 A1 describe a measuring system in which the displacement of a first assembly is measured relative to a second assembly. Two gratings are provided which are at a constant distance from each other, each of which being secured to one assembly. If the second grating receives divergent light from a light source, the first grating generates a periodic image of the second grating, this image being in motion if there is relative motion between the two assemblies. Furthermore, photodetectors are provided which have a periodic pattern and are fixedly connected to the second assembly. The first grating is a reflecting grating, and the second grating and the photodetectors are located essentially in one plane. The light source and the second grating can also be replaced by a patterned light source that generates the same image as a conventional light source and a grating. The periodic photo detectors interact with the image such that a periodic modification of the output signal of the photodetectors arises if a relative motion occurs between the first and the second assembly.
One disadvantage of the system is that it is not disclosed how the described system is to be realized. In addition, it is disadvantageous that the photoelements are always arranged on the side next to the light source, resulting in an increased sensitivity to tilting on the part of the scanning head. Furthermore, is not clear from this publication that the second grating and the patterned photo detector are placed at the same distance from the first grating, to make possible an optically optimal imaging. This apparatus is also very difficult to manufacture.
German Patent Application 197 01 941 A1, describes a scanning grating arranged on the side of a transparent carrier that is facing a scale. The scanning grating receives light from a light source such that an image of the grating is projected onto the scale. On the scale, there is located a second grating, which reflects the image onto a patterned photodetector. The transparent carrier for the first grating is bonded to the semiconductor material in which the patterned photodetector is realized, such that the scanning grating and the photodetector are staggered in the measuring direction with respect to each other, although they are at the same distance from the scale. In a second specific embodiment of German Patent Application 197 01 941 A1, the scanning grating is arranged on the side of the transparent carrier that is facing away from the scale. On the same side as the scanning grating, an optical chip is arranged on the same transparent carrier that contains the photodetector. These arrangements also assure that the scanning grating and the patterned photodetector are approximately the same distance from the scale.
In the first specific embodiment, a disadvantage arises because the transparent carrier on which the scanning grating is applied has to be bonded to the semiconductor material in which the patterned photodetector is realized. This bond must be effected very precisely so that the pattern of the photodetector is aligned parallel to the grating and the pattern and grating are the same distance from the scale. This precise bonding between carrier and semiconductor material is therefore very difficult to realize. Furthermore, the second specific embodiment has the disadvantage that an optical chip has to be secured on the transparent carrier. As a result of mounting it using chip-on-glass technology, there inevitably arises a gap between the optical chip and the carrier, as a result of which the distances between the scanning grating and the scale and between the photodetector and the scale significantly diverge from each other, which leads to a marked deterioration of the optical properties of the apparatus.
German Patent 40 91 517 T1, describes making a scanning head for a measuring system from one single block of semiconductor material. Provision is made on the surface of a planar light-emitting diode for photoelements that are configured as grating lines, through which the light from the light-emitting diode cannot penetrate. As a result, there arises a patterned photodetector, over and under which a patterned light source is arranged. As a result, the pattern of the light source and the patterned photoelements are made to have the same distance from the scale.
The scanning head, however, has the disadvantage that the photodetector pattern receives divergent light directly from the light source, and as a result a high background signal level arises in the photoelements. This is unavoidable even if the photoelements are protected on the reverse side by a metallic layer, since the edges of the photoelements are also sensitive. A further disadvantage is that the photoelements have to be applied to a carrier substrate as thin layers, in a very cumbersome process that usually does not result in high quality photoelements.
PCT/European Patent 98-04658 of the applicant describes a measuring device that, in addition to periodic incremental signals, also delivers at least one reference pulse signal, given a defined reference position of a scale and a scanning unit that is movable in relation thereto. On the scale, at least one reference marking field is arranged so as to be integrated into an incremental graduation, and the scanning unit includes a detector apparatus that has, in the measuring direction, at least three active detector areas, arranged so as to be adjoining. Two of the detector areas are used as reference pulse signal detectors and generate a reference pulse signal, the relative location of the reference pulse signal detectors in the measuring direction depending on the pattern of the reference marking field on the scale. A detector arranged between the reference pulse signal detectors functions to generate at least one incremental signal. In addition, provision is made for various measures for filtering the incremental signal component from the reference pulse signal.
German Patent Application 197 20 300 A1 describes an electronic hybrid component in which, in a chip-on-chip arrangement, an implanted chip is arranged on a carrier substrate. The carrier substrate has at least one cavity, in 30 which is located an electrical insulating layer having a metallic layer on top of it. The chip that is implanted in the cavity is contacted to the metallic layer, as a result of which the latter is used as an electrical lead. If the implanted chip is a light-emitting diode, the metallization layer can also be used for the purpose of reflecting its light beam on the walls of the cavity.
This apparatus has the disadvantage that both the beam direction of the light-emitting diode as well as its electrical contacts are arranged on one side of the semiconductor substrate or are emitted on this one side.
European Patent 543 513 A1, describes both a patterned photodetector as well as a patterned light source in the form of at least one light-emitting diode of a scanning head, formed on a common semiconductor substrate made of III/IV semiconductor material, such as gallium arsenide Ga-As. By forming the patterned light source and the patterned photodetector on a common semiconductor material, the design satisfies the requirement that the transmitting and receiving pattern be formed in one plane. Furthermore, a single-field scanning takes place, in which the photoelements are offset by xcex1+k* 360xc2x0, where k is a whole number and xcex1 is preferably 90xc2x0, 270xc2x0, 120xc2x0, or 240xc2x0. Therefore, a plurality of photoelements is arranged staggered in the measuring direction, with respect to each other, by 90 degrees of angle plus a whole-number multiple of 360 degrees of angle. In this way, the scanning becomes particularly immune to interference.
One disadvantage of this design is that no account is given as to how the patterned photodetector and the patterned light source are to be manufactured on a common semiconductor material made of Ga-As. If the technologies of semiconductor production known from the related art are used, then this manufacturing process is very cumbersome and thus expensive, since thick epitaxial layers having varying aluminum concentrations must be applied next to each other, by being pattern-imprinted onto the semiconductor substrate. Only in this way can the emission wavelength of the light-emitting diode be shifted with respect to the absorption edge of the photoelements such that significant signal currents can be generated.
There is thus a need for a scanning head and a method for manufacturing it, in which transmitting and receiving gratings and patterned transmitting and receiving assemblies provided on the scanning head are at an identical distance from the scale, and where the transmitting grating can be arranged in the center with respect to the receiving gratings. In addition, single-field scanning is to be used. The manufacturing process for the scanning head should be also cost-effective. The scanning head of this type is to be used especially in angle and in length measuring systems.
In one aspect, the invention is a scanning head for scanning a scale which carries a graduation, where the scanning head includes a semiconductor substrate having a first side facing the scale, a plurality of patterned photodetectors disposed on the first side, and a blind hole formed in the semiconductor substrate. The device also includes a transmitting grating disposed on the first side facing the scale, said transmitting grating being symmetrically aligned with respect to the patterned photodetectors, and a light source disposed on a side of the transmitting grating facing away from the scale.
In another aspect, the invention is a method for manufacturing a scanning head for scanning a scale which carries a graduation, including the steps of diffusing pn-junctions into a first surface of a semiconductor substrate facing the scale, to form photo diodes, generating an oxide layer on the first surface of the semiconductor substrate, and etching with an anisotropic etching process down to the oxide layer a second surface of the semiconductor substrate adjacent to the photo diodes, the second surface facing away from the scale.
The apparatus according to the invention has the advantage that the pattern and the receiving grating for the photodetector, as well as the pattern and the transmitting grating for the light source, are disposed at substantially the same distance from the scale to be scanned. In this way, optimal optical properties of the scanning head can be assured. Furthermore, it is advantageous that the patterns and gratings are produced in method steps which are well-known from semiconductor technology. In this way, they can be executed very precisely and very cost-effectively, with only little wastage. A further advantage rests in the fact that the photodetectors and the light-emitting diode used as the light source can be manufactured from different semiconductor materials. It is particularly advantageous that the scanning head according to the invention can be used in precision measuring systems for angle and length measuring, systems that are used preferably in numerically controlled machines.